Which of the following sets of vectors $\displaystyle \alpha=(a_1,a_2,a_3,....,a_n)$ in $\displaystyle R^n$ are subspaces of $\displaystyle R^n$ (n $\displaystyle \geq3)? $ All $\displaystyle \alpha$ such that

(a) $\displaystyle a_1+3a_2=a_3$

(b) $\displaystyle a_1a_2=0$

(c) $\displaystyle a_2=(a_1)^2$

(d) $\displaystyle a_2$ is rational